1. Technical Field of the Invention
The present invention relates to wireless telecommunications and, more particularly, to multi-path interference cancellation in a high speed data system using transmit diversity.
2. Description of Related Art
A phenomena that reduces the efficiency of a communications link is fading. Fading may take several forms, one of which is referred to as multi-path fading. Multi-path fading is caused by two or more copies of a transmitted signal combining at the receiver in a way that reduces the overall received signal level. One technique developed for reducing the effects of fading is transmit diversity. Generally, for transmit diversity, a radio transmitter uses two transmit antennas that are positioned far from each other to transmit one signal. Typically, the two transmit antennas are positioned more than several wavelengths of the transmitted signal from each other depending upon the environment.
For example, in the well known Space-Time Transmit Diversity (STTD) system, symbols S1 and S2 are received for transmission at the transmitter encoder in which S1 is received during the period from T0 to T1 and S2 is received during the period from T1 to T2. At a first output of the encoder, S1 is output for transmission to a first antenna during the symbol time from T1 to T1, followed by symbol S2 from symbol time T1 to T2. A second output of the encoder outputs the negative complex conjugate of symbol S2 for transmission to a second antenna during time T0 to T1, followed by the complex conjugate of symbol S1 from the period T1 to T2.
Another improvement, known as High Speed Downlink Packet Access (HSDPA), has been developed to enhance mobile services for high-speed data users. HSDPA takes advantage of link adaptation such as adaptive modulation and coding to enhance data rates to data users in a time-multiplexed manner. HSDPA is specified by in Third Generation Partnership Project (3GPP); Technical Specification Group Radio Access Network; Physical layer aspects of UTRA High Speed Downlink Packet Access (Release 4), the description of which is hereby incorporated by reference. The outcome of the 3GPP work is a set of specifications defining the 3G-network functionality, procedures and service aspects. HSDPA transmissions are performed on physical channels shared by other users generally employing different spreading gains. The HSDPA channels employ a spreading gain of 16 and voice users, for example, typically employ a spreading gain of 64.
HSDPA can be transmitted in a transmit diversity manner with other user transmissions. However, some conventional signal processing methods cannot be used for receivers of HSDPA data encoded in a transmit diversity scheme and, thus innovative processing methods must be developed.
For example, HSDPA can be encoded for STTD transmission on a shared channel with voice users in which the voice users may or may not be STTD encoded. In either case, however, because the spreading gain of the voice users is different from the HSDPA channels, the effective channel seen by the voice users is different from the channel seen by the HSDPA users. If STTD encoded according to Third Generation Partnership Project (3GPP); Technical Specification Group Radio Access Network; Physical channels and mapping of transport channels onto physical channels (FDD) (Release 1999), the description of which is hereby incorporated by reference, the STTD encoded data itself does not see a time invariant channel and, thus chip level equalization cannot be employed to remove the multi-path interference.